Tour de Costume

I don't know much about bike racing, like the Tour de France.  I know a bit more after watching phase 7 of the Tour of California while eating at a local restaurant.  For some reason, fans in funny costumes like to run along side the racers, at least near the end of the race.  The costumes are hilarious, its better than reality TV.  But, I don't understand why you'd watch a bike race dressed in a toga or run alongside the racers dressed in scrubs.  Do you?